1. Field of the Invention
The invention pertains to a method for the production of an anode for X-ray tubes, consisting of a base element and a coating that emits X-ray radiation, the coating differing from the base element.
2. Description of Related Art
To produce X-ray radiation, materials that emit X-rays when impacted by a focused electron beam are used. The high-melting-point metals tungsten and molybdenum and their alloys, for example, are materials typically employed, depending on the desired type of X-ray radiation.
In medical diagnostics, rotary anodes are often used for X-ray tubes in the form of axial-symmetrical disks for the generation of X-ray radiation. In most cases, only one portion of the surface is designed in the form of a ring-shaped path--the so-called focal track--in the region directly impacted by the electron beam. This focal track uses a comparatively thin coating of material to generate the X-rays. The base element of the rotary anode consists of other high-melting-point materials.
The specific material properties of the coating, such as lattice, thermal conductance, thermal expansion, mechanical properties, and thickness are important factors for the practical behavior of the focal track coating during X-ray operation. A permanent residual porosity has an adverse effect on the thermal conductance, the fatigue crack resistance, and the gas evolution in the X-ray tube. This means that the greatest possible density values are desirable for the comparatively thin focal track coating. A reduced fatigue crack strength is primarily expressed by a greatly increasing roughening of the focal track with increased use, and by a reduced X-ray yield associated with the fatigue cracking.
The focal track coating has been produced to this time primarily by means of powder-metallurgical methods (e.g., pressing, sintering, and forging). In the case of metallic materials used for the base element, the coating is primarily produced in one working step of the base element by coating the base element with the powder mixtures. In this manner, density values for the coating of 96-98% of the theoretical density are attained as the standard. A production method of this type for the focal track coating is low in cost, but results in properties that are not the optimum, particularly with regard to fatigue crack behavior.
In particular, where graphite is used as the base element material, the linkage of the base element with an independent focal track coating, produced by powder metallurgy is difficult. In contrast to powder metallurgy, a focal track coating may also be applied by known deposition coating methods, preferably by chemical vapor deposition (CVD) or even by physical vapor deposition. Of course, with vapor deposition, densities of nearly 100% of the theoretical density can be attained for the focal track coating. However, due to significantly greater manufacturing costs, vapor deposition production methods have been generally limited to production of rotary anodes with graphite bodies. Vapor deposition has not been able to replace the method of powder metallurgy for manufacturing the focal track coating.
As a somewhat lower-cost coating method with a number of processing advantages, the method of conventional plasma spraying can be used. This applies in particular when the method is applied under a controlled atmosphere, i.e., under a vacuum or inert gas atmosphere. In conventional plasma spraying, the material for the focal track coating is radially introduced as a powder into a plasma beam generated by a dc arc discharge, then melted in the plasma beam and the molten droplets are deposited on the base element. Some important advantages of this method are the high application power per unit time, a coating temperature that is adjustable over a broad range, and the avoidance of chemical compounds that are difficult to neutralize. However, in the conventional plasma spray method, in spite of intensive development efforts around the world in recent years, focal track coatings with a maximum density of only 93% of the theoretical density have been attained. These densities provide unsatisfactory results for rotary anodes coated in this way with regard to fatigue cracking and gas evolution properties. The known thermal post-treatment methods that can be used, in theory, to obtain an increase in the density have only limited lid effectiveness in practice, or they have only limited applicability due to the effects on the material of the base element and/or on the composite behavior. This applies in particular to the use of graphite as a material for the base element. Under these restricting conditions, the thermal post-treatment to achieve post-compression will not be sufficient and complete gas evolution of the focal track coating will not occur. Due to these disadvantages, manufacture of rotary anodes, in which the focal track coating was applied with conventional plasma spraying is not common.
Existing methods of powder metallurgy, vapor deposition and conventional plasma spraying have failed to achieve satisfactory coatings at low cost.